Cabin noise in small aircraft is a combination of noise from a variety of sources, the major ones being the engine, wind, and propeller (propellers are noisier on multi-engine planes where the propellers are close to the wing--noise is generated each time a prop blade passes the leading edge of the aircraft wing). Such aircraft cabin is sufficiently loud that small plane pilots routinely wear noise attenuating headsets. Such headsets usually employ passive noise attenuation in the form of an annular cushion carried on the rim of each earcup. The cushion is sufficiently flexible to conform to the pilot's head, thereby creating an acoustical seal around the pilot's ear. Such passive noise attenuation headsets--frequently referred to as circumaural headsets--can be quite effective for higher frequencies (e.g., above about 500 Hz), but typically are much less effective at lower frequencies.
Analog active noise cancellation headsets have been commercialized to reduce the noise level within headset earcups. Examples of such headsets can be found, e.g., in U.S. Pat. Nos. 5,182,774, 4,953,217 and 4,644,581. These analog systems utilize a noise detection microphone (sometimes referred to as an "error microphone") mounted within the earcup. The noise signal detected by the microphone is provided to an inverter which generates a noise cancellation signal provided to the speaker in the earcup. The acoustical pressure in the earcup is then the sum of the external noise that has penetrated the earcup and the acoustical output of the speaker. The speaker output, thus, is intended to exactly cancel the noise. Additional radio, intercom or other desired signals may also be provided to the speaker, as desired.
Commercial implementations of such analog systems have generally been capable of reducing noise by about 10-15 dB for frequencies of up to about 500 Hz, the range in which noise cancellation is most needed. These noise reductions systems have the advantage of being effective for random, unpredictable noises, as well as broadband noise (such as wind noise) but generally are capable of only about 10-15 dB of reduction. Attempts to make larger reductions require increasing the gain of the cancellation signal, and generally result in instabilities (best solved by reducing the feedback loop gain). While such broadband noise reduction is helpful, as can be seen from FIG. 1, the loudest noises in the low frequency range of a typical small plane cabin, even if attenuated by 10-15 dB, are still very loud.
Digital signal processor (DSP) based adaptive feedback cancellation systems have been proposed for dealing with certain kinds of noise that, though not necessarily constant are predictable. In these systems, an algorithm is used to construct, in real time, an adaptive filter which is used to generate the noise cancellation signal. In applications where these systems have been successfully employed the predictive nature of a tone has enabled such systems to achieve very significant levels of noise reduction--as much as 60 dB. While such dramatic noise reductions are possible, these systems typically are not sufficiently responsive to deal with transient or random noises. Rather, their effectiveness is dependent on the existence of a relatively predictable noise, such as is found in tonal noise, to which the system can adapt and make generally accurate predictions (hence, the word "adaptive"). Thus, the effectiveness of these systems is also constrained to a narrow bandwidth--i.e., they generally work well only on tonal noises which, by definition, have a narrow bandwidth (see, e.g., U.S. Pat. No. 5,546,467, showing use of a DSP active noise cancellation system to cancel a narrow noise band at about 400 Hz, produced by the fan of a hair dryer).